Field of the Invention
The present invention is in the technical field of material “segmenting” and particularly in the field of hazardous material segmenting and/or segmenting in hazardous environments, such as nuclear power plant component and equipment dismantling, or any application where metallic components are to be segmented for removal and disposal. More particularly, the present invention is in the technical field of remotely segmenting radioactive and non-radioactive nuclear reactor vessels and internal components thereof, and all related nuclear components such as piping and tanks, vessels, and spent nuclear fuel storage canisters. The invention is applicable to segmenting in air or underwater, carbon steel and stainless steel or stainless steel-clad carbon steel, which are typically found in reactor vessels and internals. The technology is generally applicable to cutting any electrically-conducting material.
History of Segmenting Technologies
Prior segmenting activities primarily relied upon electric plasma arc cutting torches, Electric Discharge Machining (EDM), or mechanical circular or band saw cutting, and high-pressure Abrasive Water Jet (AWJ) cutting equipment. These electric and mechanical tools are slow, have limited capability for cutting thick materials, generate potentially explosive gases including hydrogen, and generate secondary waste streams from the “dross” (very fine particulate removed from the “kerf”—the actual cut) thereby obstructing underwater clarity and vision required to accomplish the segmentation. The fine particulate dross is highly radioactive and is suspended in underwater cutting pools and vessels, causing unacceptable exposure of radiation to workers above the cutting pools controlling the tool position.
The mechanical cutting tools of circular or band saws are the slowest of any cutting method, having a cutting speed of less than two (2) square inches per minute (thickness times length per minute). Mechanical cutting generates secondary waste in the form of generally large particle “swarf” (the material removed from the kerf in mechanical cutting), which has proven difficult to capture at the source. It has required, for example, a separate subsequent underwater vacuuming system to collect and dispose of the swarf. The circular and band saws have a short life expectancy as the teeth wear away or become dull, requiring frequent blade changes which can take anywhere from one (1) to three (3) hours per change. Tool reaction forces and tool chatter are also major concerns requiring massive, rigid support systems (manipulators) to accurately position and hold the blade in the kerf. Earlier attempts to use mechanical cutting by means of saws to segment large metal objects were laborious, slow and time-consuming, and consumed cutting tools, normally saw blades or milling cutters, at a rapid rate. Consequently, mechanical saw blade or milling cutter segmentation is quite expensive. Prior experience using abrasive saws or cut-off wheels have proven similarly slow and dangerous as the abrasive wheel is subject to breaking and flying off at the high speed required to accomplish the cut, potentially injuring workers.
Flame cutting using oxygen-acetylene, oxygen-propane, or oxygen-gasoline torches or burners are faster, but do not burn hot enough to penetrate stainless steel materials effectively. They generate large quantities of gases that must be controlled, especially when cutting radioactively contaminated or activated materials like nuclear reactor vessels. Similarly, high-temperature plasma arc cutting torches have been used wherein a high voltage arc coupled with an inert gas creates a high temperature plasma to melt the metal from the kerf. These plasma arc torches are faster (five (5) to twelve (12) square inches a minute) than mechanical or AWJ cutting, but generate large quantities of hydrogen and contaminated gases, and the particulate from the kerf clouds the cutting pool water obstructing visibility.
EDM uses a focused electric arc to melt metal similar to the arc saw, but the arc beam is not sufficiently powerful, nor fast enough to be used for the much larger metal thicknesses encountered in reactor dismantling. The cutting speed is much less than one (1) square inch per minute.
Another technology applied relatively recently in the nuclear industry for decommissioning reactor vessels and internals is high-pressure AWJ cutting. The process uses a high-pressure pump (called an intensifier) to generate water pressures of between forty thousand pounds per square inch (40,000 psi) and sixty thousand pounds per square inch (60,000 psi), directed through a jet nozzle into which is also injected by high-pressure air an abrasive grit such as garnet. The combination of water pressure and grit erodes the metal in the kerf (areas being cut). This method has been used for both nuclear reactor vessels and internals, on both carbon steels and stainless steels. However, the process is exceedingly slow at less than two (2) square inches of material per minute (comparable at best to mechanical saw cutting), and generates an exorbitant amount of secondary waste in the form of contaminated water and grit, mixed with the radioactive material removed from the kerf. As an example, a typical AWJ will consume approximately five (5) to eight (8) gallons per minute of water and one (1) to two (2) pounds of grit per gallon of water, resulting in five (5) to sixteen (16) pounds of grit per minute of cutting. To cut a two (2) inch thick plate for a distance of twelve (12) inches would take twelve (12) minutes, and generate sixty (60) to ninety-six (96) gallons of water and sixty (60) to one hundred and ninety-two (192) pounds of grit. For carbon steel and a one-eighth (⅛) inch wide kerf, these grit volumes represent seventy (70) to two hundred and twenty-three (223) times the amount of material removed from the kerf. The spent grit is co-mingled with the radioactive swarf of the kerf, and requires special packaging for transport and disposal to a licensed radioactive disposal facility. The grit cannot be recycled. AWJ grit emanating from the kerf disintegrates into a fine particulate which obscures underwater clarity and vision, requiring frequent cessation of segmenting operations to allow pool or vessel filtering systems to clean up the water. Control and collection of this radioactive grit as it exits the kerf has been a serious problem for the nuclear industry, requiring extensive post-cutting collection and cleanup of the cutting pool or reactor. Although the AWJ reaction forces are not as large as in mechanical cutting, the tool still requires a rigid support manipulator to position and maintain the jet in the kerf.
None of the foregoing cutting technologies can effectively cut through the multiple thicknesses frequently encountered in reactor vessel and internals segmentation. Plasma arc torches and EDM can only maintain their arc over a short distance and thickness, and cannot extend through multiple thicknesses of material. Circular and band saws encounter tool chatter causing rapid loss of sharpness and tool teeth from the blade. The exit spray from the kerf of the AWJ cutters fans out from the first material thickness encountered, and loses its ability to concentrate the jet to pierce through the second or more thicknesses of the cut.
History of Arc Saw Development
The arc saw was originally proposed and patented in various forms as a machining tool in U.S. Pat. No. 2,015,415 in 1935, U.S. Pat. No. 2,059,236 in 1936, and U.S. Pat. No. 2,355,838 in 1944 (de-surfacing/machining devices), U.S. Pat. No. 4,243,862 (an arc cutting device), and U.S. Pat. No. 4,608,477 in 1986 (a hand held electric arc saw apparatus). The latter hand-held arc saw would not be suitable for the highly radioactive components of nuclear reactor vessels, internals or similarly radioactive components owing to the excessive radiation exposure to the worker.
The most recent arc saw patents (known to Applicant) are U.S. Pat. Nos. 4,401,875 and 4,463,242 (hereinafter called the “Retech Patents”; as they are both assigned to Retech, Inc. of Ukiah, Calif.) describe in detail the actual operation of a standard arc saw, the voltage and current required, and the quality of the kerf.
The arc saw described in the Retech Patents comprises a toothless carbon steel circular blade supported on an arbor (shaft) and driven by an oil-hydraulic motor. The blade is connected to a low-voltage, high-amperage electric power supply through commutators mounted on the blade rotating head. The power supply provides twenty-five (25) to fifty (50) volts of Direct Current (DC) and an amperage of fifteen thousand (15,000) to twenty-five thousand (25,000) amperes to the blade to melt the material being cut (hereinafter, the “work piece”). The blade does not touch the work piece, and servo-motors are used to maintain an arc of eight (8) to sixteen (16) thousandths of an inch gap using an electrical feedback control circuit and opposing force servo-motors (to eliminate slack) based on maintaining a constant amperage to the work-piece. There are virtually no reaction forces between the blade and work-piece other than the small amount of friction of the blade through the water used for cooling the blade. The blade of the Retech Patents, in practice, encountered difficulties with side-arcing of the blade to the work-piece, causing rapid degradation of the blade and in some cases distortion of the blade due to heating from the side arc and magnetic distortion from the high current.
The authors of the Retech Patents built a prototype for testing, and developed the mathematical relationships empirically for cutting current versus thickness of material to be cut, and the speed of cutting. The final equation the authors developed is as follows:v=[I/s−J0]1/M Where:
v=cutting speed in m/sec
I=the cutting current in amps
s=w times I in m2=the effective surface area of the kerf or cut=width times length
J0=threshold current density=a×106 amperes per square meter, wherein “a”=2 to 6
M=the change of the current density divided by the change in the velocity
The two original authors of the Retech Patents ran more than six hundred (600) tests of the arc saw to develop these cutting parameters and this cutting speed relationship. Their findings are generally accepted in arc saw design.
The performance of the arc saw of the Retech Patents is indicated in the following example. Using a blade of approximately one-quarter (¼) of an inch thick, and an operating voltage of twenty-five (25) volts, a cutting current (“I”) of thirteen thousand (13,000) amps, a cutting speed of approximately two hundred and seventy-nine inches squared per minute (279 in2/min) was achieved in carbon steel, and seven hundred and seventy-five inches squared per minute (775 in2/min) in aluminum. These cutting rates in steel are fifty (50) to one hundred and forty (140) times faster than mechanical cutting, plasma arc or AWJ cutting. Higher rates are achievable using higher cutting amperages.
The Retech Patents overcame some of the much earlier difficulties of an inability to sustain the cutting action of the arc saw, rapid wear of the blade, unexpected current surges and voltage variations resulting in unreliable cutting speeds. They also designed a blade configuration to include gullets around the blade periphery to permit removal of the cutting debris (dross) from the kerf. These gullets also served to cool the blade when immersed in water. However, side arcing of the blade to the work piece was still a problem with rapid blade wear and consumption. This arc saw required the blade to be completely immersed in water for cooling and removal of the dross. In-air cutting without cooling would rapidly degrade and distort the blade resulting in a short blade life and frequent blade changes. The blade material for the Retech Patents blade was carbon steel, which while less expensive is subject to magnetic distortion (bending) from the high currents between the work-piece and the blade, particularly when cutting carbon steel. This distortion also contributed to short blade life. This design was not capable of rapid blade changes which is advantageous in reactor vessel and internals segmentation.
The Retech Patents arc saw invention found limited acceptance in the nuclear decommissioning industry, although the Japanese applied the principle to segmenting the reactor vessel at the Japan Power Demonstration Reactor, and in the U.S., Argonne National Laboratory West used it to segment parts of nuclear fuel assemblies to prepare them for shipping and disposal. Other companies offering mechanical or electrical segmentation services apparently were unfamiliar with the arc saw, and had already committed their financial and technical personnel resources to the other aforementioned technologies as they knew these technologies best. Companies were also generally afraid to take a chance on a new technology, and hadn't spent the time to research where the arc saw could be used successfully. They may also have been concerned about the reports of the “rough cut” surface produced by the existing arc saw.